<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
 
 *{
  margin: 0;
  padding: 0;
 }
 .box{
 }
  .parent{
    height: 100px;
    width: 100px;
    background-color: red;
    position: relative;
    z-index: auto;
  }
  .son{
    height: 100px;
    width: 100px;
    position: absolute;
    background-color: blue;
    z-index: -1;
    top: 10px;
    left: 10px;
  }
</style>
<body>
  <div class="box">
    <div class="parent">
      <div class="son"></div>
    </div>
  </div>
  
</body>
</html>